Denitrification of nitrate in regeneration waste brine using hybrid cation exchanger supported nanoscale zero-valent iron with/without palladium nanoparticles

Abstract

The Sustainable Development Goals require that reducing waste is a priority. This work described the application of an innovative zero-waste hybrid ion exchange nanotechnology that concurrently removed nitrate and induced denitrification to ammonia, with the ability to generate fertilizer for the agriculture sector from the recycled by-products. Herein, hybrid cation exchanger-supported zero-valent iron (Fe⁰), and bimetallic Fe⁰/Pd nanoparticles (HCIX-Fe⁰ and HCIX-Fe⁰/Pd) were synthesized and successfully validated for denitrification of nitrate in spent waste brine that contained nitrate. The kinetics of nitrate catalysis by both HCIX-Fe⁰ and HCIX-Fe⁰/Pd were compared and presented by six kinetic models, namely, zero-order, pseudo first- and second-order reaction, pseudo first- and second-order adsorption, and Elovich. HCIX-Fe⁰/Pd displayed a higher kinetic value than HCIX-Fe⁰, with k₁ of 0.0019 and 0.0026 min^-1, respectively. Nitrate was predominantly catalysed to NH₄⁺ at a ratio of ammonia to other nitrogen compounds of around 80:20. Although HCIX-Fe⁰/Pd showed slightly better (14%) kinetic results, it was determined as unfavourable for real-life application due to low selectivity toward N₂ gas and the need to use H₂ gas. Based on practicability, the HCIX-Fe⁰ was further validated. The effect of salt (using NaCl) and the role of initial pH conditions were optimized and discussed. The recovery of nitrate removal was also calculated, and a recovery range of 91.42-99.14% was obtained for three consecutive runs. The sustainable, novel, zero waste hybrid ion exchange nanotechnology using the combination of two fixed-bed columns containing nitrate-selective resin for nitrate removal and novel HCIX-Fe⁰ for nitrate reduction to NH₄⁺ may be a promising sustainable solution toward the goal of discharging zero nitrate waste to the environment.

Keywords: Bimetallic Fe(0)/Pd nanoparticles; Catalytic nitrate reduction; Cation exchanger; Nanoscale zero-valent iron; Zero liquid discharge nitrate removal.